As the global number of Covid-19 cases heads towards 2.5 million, doctors and scientists are facing many unexpected challenges.
Key among them is how the virus works, how to treat the infection and how to end the lockdowns safely.
Here, The National breaks down five conundrums the coronavirus outbreak continues to throw up.
Is being male is a major risk factor for death from Covid-19?
As the pandemic spread from China across Europe to the rest of the world, the elderly and those with underlying health issues quickly emerged as being especially at risk.
But statistics from many countries now point to another key risk factor: being male.
It is not simply that more men are becoming infected: the data suggest they are typically no more likely to contract Covid-19 than women.
But once they do, men appear to be around 70 per cent more likely to die as a result – and around twice as likely in Italy and the Netherlands.
Gender differences in death rates from viral diseases have been seen before – notably during the Sars and Mers epidemics.
But the exact cause is unclear. One leading theory is that men are generally less healthy than women, and have higher rates of proven risk factors such as high blood pressure, cardiovascular disease, diabetes and obesity. When researchers correct for these factors, the disparity shrinks.
But another explanation is that women are genetically better-able to fight disease.
Some key immune system genes - including one that helps combat coronaviruses - lie on the X chromosome, of which women have two copies, while men have only one.
Whatever the explanation, it’s clear that gender needs to be taken into account when developing a vaccine, as half the world’s population may react differently to it than the other half.
Is there a link between ethnicity and risk of dying from Covid-19?
As the pandemic took hold in the United States, bizarre social media claims circulated suggesting African Americans were immune from Covid-19.
Within days, this was debunked in the starkest terms, as evidence emerged that the community is at significantly greater risk of death.
While the statistics remain patchy, an investigation by the Associated Press suggests that around 30 per cent of Covid-19 deaths in the US are among African Americans, despite making up half that proportion of the population.
Bitter arguments have broken out about the causes. The US Surgeon General Dr Jerome Adams - himself an African American - attracted opprobrium for publicly urging African Americans to stop drinking, smoking or taking drugs to better protect themselves against the disease.
His critics pointed to demographic issues ranging from lack of access to healthcare to poor housing standards as key drivers of the disparity.
The greater prevalence of known Covid-19 risk-factors such as diabetes and obesity has also been cited.
A similar debate has broken out in the UK, where the majority of healthcare workers who have died from the virus are from ethnic minorities.
A key issue is the lack of specific statistics. One possibility is that the higher death-rate reflects the higher proportion of those in harm’s way being from ethnic minorities.
Does Vitamin D help defend against infection?
While usually linked with bone health, vitamin D is known to interact with the so-called B cells and T cells that play a key role in the disease-fighting immune system.
The possibility that vitamin D can protect against respiratory viruses is backed by evidence that seasonal flu outbreaks tend to occur in autumn and winter, when levels of sunlight – vital for the body’s creation of vitamin D – are relatively low.
Some studies have also found that taking vitamin D supplements – around 10 micrograms per day – may help protect against viral infection.
Public health organisations in some countries are now recommending taking supplements during lockdowns to make up for a lack of sunlight.
But researchers at the University of Granada, Spain, believe the vitamin D may also help combat the disease after infection.
They are now setting up a trial involving 200 people with mild forms of Covid-19 to see if it can halt progression of the disease.
Could ventilators be doing more harm than good?
Seen as vital to saving the lives of critically ill patients, ventilators keep the oxygen levels in their blood high enough to prevent organ damage and death.
But they are very blunt instruments, requiring patients to be heavily sedated and a breathing tube inserted deep into their wind-pipe.
Now some clinicians claim ventilators are being used too often, and that much simpler, gentler methods like continuous airway pressure (CPAP) devices are enough for many patients.
This follows suspicions that the plunging oxygen levels caused by Covid-19 may be less threatening than originally thought.
According to reports, many Covid-19 patients have apparently lethally low levels of blood oxygen, yet are not gasping for air or showing signs of organ failure.
Some clinicians believe this shows the patients are more like those suffering from altitude sickness than pneumonia.
Forcing oxygen into the lungs of such patients using a ventilator could do more harm than good by damaging lung tissue.
While insisting that ventilators are still needed for many patients, researchers are now investigating the wider use of CPAP and similar procedures.
Is there a scientific consensus on face-masks?
With several countries, among them Germany, Norway and the Czech Republic, gradually lifting restrictions, questions remain over whether and when face-masks should be worn by the public.
Many countries are looking to the scientific community for guidance, only to discover there is no consensus.
From the type of mask and how to correctly fit it to the risks of careless removal and disposal, rarely has so “obvious” a measure proved so contentious.
Over the years, many studies have been carried out into the effectiveness of using masks to protect against viral infections.
Their results have been pulled together in so-called systematic reviews designed to give policy-makers clear guidance.
Yet even the World Health Organisation and US Centres for Disease Control and Prevention have been unable to reach a clear verdict.
Reviewing the evidence in a leading UK medical journal, a team led by Professor Trish Greenhalgh of the University of Oxford argued masks should be worn in public because "we have little to lose and potentially something to gain".
Robert Matthews is Visiting Professor of Science at Aston University, Birmingham, UK